Vacuum flask



FIG. 1 is a front elevational view of a vacuum flask showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a left side view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is an enlarged top plan view thereof;

FIG. 6 is an enlarged bottom plan view thereof; and,

FIG. 7 is a perspective view thereof.

The broken lines in the drawings depict portions of the vacuum flask that form no part of the claimed design. 

CLAIM The ornamental design for a vacuum flask, as shown and described. 